Enhanced recovery of oil from a dipping subterranean oil-bearing reservoir using light hydrocarbon and carbon dioxide

ABSTRACT

Recovery of oil from a dipping subterranean oil-bearing reservoir is effected by the injection of a first slug of a light hydrocarbon at a high rate to insure mixing with the reservoir oil adjacent the injection well followed by the injection of a second slug of carbon dioxide at a low rate to form a conditionally miscible transition zone with the altered reservoir oil and thereafter by the injection of a drive agent.

FIELD OF THE INVENTION

This invention relates to the recovery of oil from a dippingsubterranean oil-bearing reservoir by the injection thereinto of a firstslug of a light hydrocarbon at a rate to insure its mixing with thereservoir oil, thereby altering the composition of the reservoir oiladjacent the injection well, followed by the injection of a second slugof carbon dioxide that is conditionally miscible with the alteredreservoir oil in the vicinity of the injection well and thereafter bythe injection of a drive agent to displace the injected fluids and thereservoir oil through the reservoir to a production well from which theyare produced.

DESCRIPTION OF THE PRIOR ART

In the recovery of oil from subterranean oil-bearing reservoirs normallyprimary recovery methods are initially employed that utilize thereservoir energy present in the reservoir in the form of water underpressure or gas either in solution or under pressure. After the primaryenergy of the reservoir has been expended, additional oil may berecovered by employing secondary methods in which energy is supplied tothe reservoir from an external source, such as the injection of water asin a waterflood.

Additional recovery may be realized by employing other recovery methodsafter a reservoir has been waterflooded to an uneconomic level. Thesesubsequent recovery procedures have been termed "enhanced recovery" or"tertiary recovery" in the art.

One of these newer methods for enhanced recovery that has been practicedis termed "miscible flooding", wherein a fluid is injected into thereservoir that is miscible with the reservoir oil at reservoirconditions of temperature and pressure. The term "miscible" as usedherein means that the injected fluid is soluble in all proportions withthe reservoir oil at reservoir conditions of temperature and pressure.

Miscible flooding is effective in stripping and displacing the reservoiroil from the reservoir matrix through which the miscible fluid flows.When miscibility exists between the injected fluid and the reservoir oilat the conditions of temperature and pressure of the reservoir, a singlephase fluid is present, and the retentive forces of capillarity andinterfacial tension are eliminated. These forces are significant factorsin reducing the recovery efficiency of oil in conventional floodingoperations, such as waterflooding, where the displacing agent and thereservoir oil exist as two phases.

Miscible flooding is normally accomplished by displacement techniqueswhereby a solvent fluid that is miscible with the reservoir oil atreservoir conditions is injected into the reservoir which fluid freesthe oil from the reservoir matrix and displaces it through the reservoirtoward a production well from which the oil is produced. Normally, thefluids used are light hydrocarbons in the range of C₂ to C₆. Inparticular, liquid petroleum gas (LPG) has been extensively employed.

Because of the expense and limited availability of the lighthydrocarbons, advances in the art have included the use of a slug of thesolvent fluid, thereby minimizing the amount of solvent required. By theslug method, a fraction of a pore volume of solvent fluid is injected,which is then followed by a cheaper and/or more available drive agent.One such material employed as a drive agent is natural gas. In the slugtype miscible flood the solvent fluid is miscible with the oil at theleading edge of the slug and may or may not be miscible with the driveagent at the trailing edge of the slug. In U.S. Pat. No. 3,354,953, forexample, a miscible slug process is taught in which a slug of liquidmiscible with the oil is injected in amounts sufficient to form a bandof substantially pure miscible liquid that is also miscible with thedrive agent that is subsequently injected.

Prior art has also recognized that miscibility may be either "firstcontact" miscibility or "conditional" miscibility. Conditionalmiscibility is distinguished from first contact miscibility by the factthat miscibility is achieved in the conditional miscibility situation bya series of multiphase contacts between the injected fluid and thereservoir oil. As to the type of miscibility obtained, this isdetermined by reservoir conditions and compositions of fluids. Methodsfor achieving one or the other type are described in the art.Conditional miscibility may also be divided into a vaporizing gas typein which intermediate components of the oil are vaporized into theinjected fluid until miscibility is attained, or enriched gas typewherein the intermediates of the injected fluid are absorbed by the oiluntil miscibility is obtained.

It has also been long known that carbon dioxide may be utilized as arecovery agent because of its ability to dissolve in the oil, therebycausing swelling of the oil and a reduction in viscosity, both of whichaid in increasing oil recovery. In one teaching employing carbon dioxidein U.S. Pat. No. 3,262,498, carbon dioxide preferably is injected in theliquid state into a reservoir where it goes into solution in the oil togive the beneficial effects of swelling and viscosity reduction, andthereafter a liquefied hydrocarbon is injected to that a transition zoneis formed to obtain an improved sweep efficiency. Subsequent to theinjection of the liquefied hydrocarbon, a drive fluid or agent isinjected to displace the reservoir fluids to a production well, fromwhich they are produced. In other developments, carbon dioxide has beensuggested as a recovery agent under conditions of miscibility with theoil, wherein a slug of carbon dioxide is injected at high pressure or atconditions of miscibility, which slug is thereafter driven by an inertgas or water.

More recently the use of carbon dioxide has been disclosed wherein thecarbon dioxide is employed at conditions of conditional miscibility withthe reservoir oil. For example, in U.S. Pat. No. 3,811,502 a zone ofconditional miscibility between the carbon dioxide and the oil isestablished, after which a drive agent is injected as the displacingforce. In another development as taught in U.S. Pat. No. 3,811,503 aslug of a mixture of a light hydrocarbon and carbon dioxide is injectedthat is conditionally miscible with the reservoir oil, which slug isfollowed by a drive agent. The amount of light hydrocarbon and carbondioxide present in the slug are in a critical ratio which ratio assuresthat conditional miscibility will exist between the slug and thereservoir oil.

The instant invention discloses an advance in the use of carbon dioxidein a miscible flood whereby, prior to the injection of the carbondioxide, the reservoir oil in the vicinity of the injection well isaltered by the injection of a light hydrocarbon at a rate to insuremixing with the reservoir oil so that the altered fluid is conditionallymiscible with the later-injected carbon dioxide. By the method ofinvention not only are the higher pressures required for utilizingcarbon dioxide alone as the miscible fluid not necessary, but alsosmaller amounts of the light hydrocarbon are effective. In addition, bymeans of the instant invention it is not required that a band ofsubstantially pure solvent be established and maintained as the floodproceeds. Nor is it required that leading edge miscibility beestablished between the reservoir oil and the injected hydrocarbonfluid.

It is thus an object of the instant invention to extend theeffectiveness of the use of carbon dioxide for in-situ oil recovery toreservoirs not previously amenable to miscible flood operationsemploying carbon dioxide because of formation pressure limitations.

SUMMARY OF THE INVENTION

This invention relates to the recovery of oil from a dippingsubterranean oil-bearing reservoir wherein a slug of a light hydrocarbonis rapidly injected into the reservoir to alter the composition of thereservoir oil in the vicinity of the well bore and thereafter a slug ofcarbon dioxide is injected at a slow rate whereby a conditionallymiscible transition zone is formed with the altered reservoir fluid.Thereafter, a drive agent is injected to displace the injected fluidsand reservoir oil through the reservoir toward a production well fromwhich they are produced.

DESCRIPTION OF THE INVENTION

This invention relates to the recovery of oil from a dippingsubterranean oil-bearing reservoir at reservoir conditions oftemperature and pressure at which carbon dioxide is soluble in, but notmiscible with, the reservoir oil.

In its broadest aspect the invention comprises introducing into adipping subterranean oil-bearing reservoir a slug of a light hydrocarbonat an injection rate high enough in amounts sufficient to form a mixtureof the reservoir oil and the light hydrocarbons in the vicinity of theinjection well. Thereafter, a slug of carbon dioxide is injected at alow rate to form a transition zone of conditional miscibility with thealtered fluid around the injection well. After sufficient carbon dioxidehas been injected, a drive agent is injected to displace the injectedfluids and the reservoir oil through the reservoir toward a productionwell from which they are produced.

The instant invention employs a first slug of hydrocarbon to conditionthe reservoir in the vicinity of the injection well prior to theinjection of carbon dioxide. The injected slug of hydrocarbon isinjected at rates high enough to cause mixing of the light hydrocarbonand the reservoir oil in the vicinity of the injection well to providean altered resevoir fluid that at reservoir conditions of temperatureand pressure is conditionally miscible with the later-to-be injectedcarbon dioxide. There is no necessity that the injected first slug becapable of miscibility with the reservoir oil. The amount of sluginjected can be determined by calculation utilizing reservoir mechanics,and the composition necessary to assure that the altered reservoir fluidhave miscibility with the carbon dioxide can be determined by laboratorytests.

The invention resides in the dipping fact that the reservoir is floodedat reservoir conditions by altering the composition of the fluid aroungthe injection well bore so that a conditional miscible carbon dioxideflood can then be conducted. The method is applicable to, but notrestricted to, reservoirs which are too low in pressure to allow forcarbon dioxide alone to have conditional miscibility with the reservoiroil. Conditional miscibility within the meaning of this invention hasbeen defined heretofore and is to be distinguished from first contact orinstant miscibility. The conditional miscibility is achieved by a seriesof transition multiphase contacts wherein the light hydrocarbons in thealtered oil zone around the injection well are absorbed into the carbondioxide thereby creating in-situ a miscible transition zone between thealtered fluid and the carbon dioxide slug.

In demonstrating the invention a series of slim tube tests was conductedusing a 40 foot long stainless steel tube (0.25" diameter). The tube waspacked with Ottawa 40-60 mesh sand. Suitable temperature and pressurecontrols and production measuring devices were employed.

In operation the sand pack was saturated with the oil of interest togive an initial oil saturation (S_(oi)) of 1.00; thereafter, thesand-packed tube was initially waterflooded to irreducible oilsaturation (S_(or-1)). The displacing fluid (or fluids) of interest wasthen injected in a predetermined amount and at a given rate and the oildisplacement was monitored by means of observing the effluent from thetube. Thereafter, a drive fluid was injected. Observation of the firstappearance of a second phase was noted in a high-pressure sight glass.Recovery, measured as residual oil saturation (S_(or-2)), was determinedat the time there was no further recovery of oil.

A series of tests using a given reservoir oil, having an API of 32°, wasconducted at a pressure of 2730 psia and a temperature of 160° F.Calculation of the size of the slug was based on the minimum miscibilitypressure correlation for carbon dioxide and oil and assuming that thefirst 4 to 5 feet of the tube was needed to establish conditionalmiscibility.

The procedure and results from the test are given in the accompanyingtable.

The results demonstrate that improved recovery can be obtained by theuse of a conditioning slug of a light hydrocarbon prior to theundertaking of a conditionally miscible carbon dioxide flood.Furthermore, the results show that high recovery can be obtained evenwhen the size of the conditioning slug is of the order of 1.5% PV (PoreVolume), which is significantly smaller than slug sizes used inconventional miscible slug floods, that are generally of the order of 3to 10% PV.

For example Run 5 in which a conditioning slug of butane was used (1.5%PV) the recovery efficiency (E_(r')) for the tertiary or enhancedportion of the run was 95.0%. A larger conditioning slug of 3% PV in Run2 gave a recovery efficiency (E_(r')) of 96.9%. These results contrastfavorably with the recovery efficiency obtained for either a carbondioxide enhanced recovery flood (Run 1) or the use of a mixture ofbutane and carbon dioxide (Run 4) in which the amount of C₄ was 3.2% PVwhich recovery efficiencies (E_(r')) were 56.4% and 79.9% respectively.

    ______________________________________                                                             RECOVERY                                                                      EFFICIENCY                                               Run  Steps In                              E.sub.r                            No.  Procedure       S.sub.or-1                                                                           S.sub.or-2                                                                         E.sub.r                                                                            E.sub.r'                                                                           Total                              ______________________________________                                             1. Waterflood   0.304       69.6                                         (1)  2. Inject CO.sub.2 (30%PV)                                                    3. Waterflood          0.133     56.4 86.7                                    1. Waterflood   0.291       70.9                                         (2)  2. Inject C.sub.4 (3%PV)                                                      3. Inject CO.sub.2 (30%PV)                                                    4. Waterflood          0.009     96.9 99.1                                    1. Waterflood   0.266       73.4                                              2. Inject mixture of                                                     (3)    (6.4%-C.sub.4)                                                              (33%PV)                                                                         (93.6%-CO.sub.2)                                                            3. Displace with N.sub.2                                                                             0.013     95.3 98.8                                    1. Waterflood   0.279       72.1                                              2. Inject mixture of                                                     (4)    (3.2%-C.sub.4)                                                              (33% PV)                                                                        (96.8%-CO.sub.2)                                                            3. Displace with N.sub.2                                                                             0.056     79.9 94.4                                    1. Waterflood   0.325       67.5                                         (5)  2. Inject C.sub.4 (1.5%PV)                                                    3. Inject CO.sub.2 (30%PV)                                                    4. Waterflood          0.016     95.0 98.4                               ______________________________________                                         ##STR1##                                                                      ##STR2##                                                                      -                                                                             ##STR3##                                                                 

The following field example further demonstrates the invention asapplied to an oil-bearing reservoir at a depth of 9100 ft, with a dip of27° and containing a 37° API oil. The reservoir was at a pressure of3788 psia and a temperature of 216° F. At reservoir conditions theconditional contact miscibility pressure is 4166 psia for CO₂ and thein-place oil. An injection well and a production well were located about575 feet apart. The reservoir had previously undergone waterflooding. Inthe test about 20,000 gallons or 0.04% PV of a butane/propane mixturewas injected at a rate fast enough to insure mixing of the hydrocarbonmaterial with the reservoir oil in the vicinity of the injection wellbore. The injection rate employed was greater than the critical velocityat the solvent-oil interface which for the particular example, wasgreater than 5 ft/day.

Calculations indicated that about 156,000 gallons of the reservoir oilhas been altered by the mixing thereinto of the butane/propanehydrocarbon slug. The altered composition was such that the pressure forconditional miscibility was about 3600 psia, as determined by the slimtube tests. After the hydrocarbon slug had been injected, a slug ofcarbon dioxide was injected at rates adjusted to balance the productionvolume. About 20% PV of carbon dioxide was injected. The injection rateof CO₂ was kept below the critical velocity for CO₂ and the reservoiroil which was about 2 ft/day. Thereafter, a drive fluid of nitrogen wasinjected to displace the reservoir fluids toward the production wellfrom which they were produced. Recovery was about 80% of the estimatedoil in the swept volume.

In the practice of the invention a first slug of a light hydrocarbon isinjected into the reservoir via the injection well to condition thereservoir adjacent the injection well by altering the composition of thereservoir oil or fluid. The hydrocarbon is injected at a rate highenough to insure its mixing with the reservoir oil. The rate should begreater than the critical velocity at the solvent-oil interface whichgenerally is in the range of 0.5 ft/day to 15 ft/day. Critical velocityis defined as the velocity at which the velocity forces become greaterthan the gravitational forces. At that point viscous fingering of thedisplacing fluid begins and thereafter continues to grow. Criticalvelocity can be determined from methods well-known in the art. Forexample, the critical velocity (v_(c)) can be calculated by thetheoretical equation: ##EQU1## where k = permeability

Δ p = density difference between displaced and displacing fluids

Δ μ = viscosity difference between displaced and displacing fluids

α = angle of dip of reservoir

φ = fractional porosity of the porous media

The amount of light hydrocarbon required can be calculated using acorrelation of minimum miscibility pressure as a function of oilcomposition and other parameters, and a minimum length of 4-5 feet toestablish miscibility. This distance applies to stable flow conditionsin dipping reservoirs where viscous fingering is absent. Generally theamount of light hydrocarbon required is in the range of from about 0.02%to about 5.0% pore volume.

As an example, a given reservoir containing a 32° API oil, with an oilsaturation of 25% and a pressure of 2700 psia and a temperature of 160°F. has the following composition:

C₁ + n₂ ; 41.27%

c₂ -c₄ ; 6.99%

c₅ +; 51.74%

using the minimum miscibility correlation, a minimum pressure of 3400psia is required to attain conditional miscibility between the reservoiroil and carbon dioxide. Again using the correlation, the minimumconditional miscibility pressure could be reduced to the existingreservoir pressure of 2700 psia. Since only 5 feet of reservoir lengthis needed to establish conditional miscibility, only the immediate areaadjacent the injection well bore need be treated. If the distancebetween the injection and production well is 300 feet, the fraction ofthe reservoir pore volume (PV) adjacent the injection well that must betreated would be: ##EQU2##

The light hydrocarbon solvent employed may be any light hydrocarbonhaving from 2 to 6 carbon atoms in the molecule. Examples are ethane,propane, LPG, butane, pentane and hexane. The solvent can also be amixture of light hydrocarbons and may contain methane, the mixture beingselected such that after having been mixed with the reservoir oil, thealtered composition is capable of forming a conditional miscible zonewith carbon dioxide. Such compositions for the hydrocarbon slug can bedetermined by means of slim tube tests such as described in U.S. Pat.No. 3,811,502.

After having established a zone of altered fluid around the injectionwell, a slug of carbon dioxide is injected at a sufficiently low rateand in amounts such that a transition zone having conditionalmiscibility is formed with the altered fluid and stable flow ismaintained. In the injection of the carbon dioxide the rate of injectionshould be less than the critical velocity at the carbon dioxide-alteredfluid interface, usually in the range of about 0.03 ft/day to about 10ft/day. The amount of carbon dioxide injected may be in the range of 10%to 30% pore volume. The carbon dioxide slug may comprise carbon dioxideor it may contain inert gas such as is taught in U.S. Pat. No. 3,811,501or light hydrocarbons such as is taught in U.S. Pat. No. 3,811,503. Theratio of the inert gas or light hydrocarbon to the carbon dioxide atwhich conditional miscibility may be attained has a critical ratio,which ratio can be determined by means in the lagoratory for exmple aslim tube test.

By an inert gas is meant a bas with a solubility in the hydrocarbonfluid with which it will be in contact of less than that of carbondioxide. Examples of inert gases are methane, natural gas, separatorgas, flue gas, nitrogen, and air and mixtures thereof. Examples of thelight hydrocarbon include ethane, propane, LPG, butane and mixturesthereof.

A drive agent is then injected to drive injected fluids and thereservoir oil through the reservoir, towards the production well fromwhich they are produced. The drive agent may be any relativelyinexpensive fluid, including gas such as nitrogen, air, combustion orflue gas, separator gas, natural gas or mixtures thereof. The driveagent may also be water, brine and/or thickeners and contain additivessuch as a surfactant to improve displacement efficiency and improve theoil recovery.

The drive agent is injected in amounts sufficient to displace thereservoir oil or fluids through the reservoir and is injected at a ratenot to exceed the critical value determined for the carbondioxide-altered oil interface so that the preferred rate of movementthrough the reservoir is from about 0.03 ft/day to about 10 ft/day. Itis within the scope of the invention to apply the method to dippingreservoirs by the injection of the carbon dioxide slug either into anup-dip injection well or into a down-dip injection well. The method ofselection in the application to dipping reservoirs is determined byreservoir conditions and the characteristics of the reservoir fluids, asfor example the density of the crude oil at reservoir temperature andpressure. The method may also be applied as a vertical displacementwherein the slugs are injected at the top of the oil-bearing reservoirand a blanket or layer of the carbon dioxide slug is established priorto the injection of the drive agent, which agent which agent displacesthe said blanket and reservoir oil downwardly through the reservoir,toward suitably placed production wells, from which the fluids areproduced.

In summary, in accordance with the practice of this invention, recoveryof oil from a dipping reservoir is accomplished by a conditionalmiscible carbon dioxide flood where prior to the injection of the carbondioxide a conditioning slug of a light hydrocarbon is injected into thereservoir at rates to cause its mixing with the reservoir oil in thevicinity of the injection well so as to form a reservoir fluid ofaltered composition, that is conditionally miscible with carbon dioxide.After a sufficient amount of the light hydrocarbon is introduced, a slugcomprising carbon dioxide is introduced at a rate less than the criticalvelocity and in sufficient amounts to establish and maintain a zoneconditional miscibility with the altered fluid. Thereafter there isintroduced a drive agent such as gas or water. The injection of thedrive agent is continued so as to displace the injected fluids and thereservoir oil through the reservoir toward a production well from whichthey are produced.

We claim:
 1. A method for the recovery of oil from a dippingsubterranean oil-bearing reservoir traversed by at least one injectionwell and one production well said oil being immiscible with carbondioxide at the reservoir conditions of temperature and pressure,comprising the steps of:(a) injecting via said injection well a firstslug of light hydrocarbon at a rate exceeding the critical velocity andin amounts sufficient to form a mixture of altered fluid of reservoiroil and light hydrocarbon adjacent said injection well, said mixturebeing conditionally miscible with carbon dioxide at the reservoirconditions of temperature and pressure, (b) injecting via said injectionwell a second slug comprising carbon dioxide at a rate less than thecritical velocity and in amounts sufficient to form a transition zone ofconditional miscibility between said mixture and said carbon dioxideslug, (c) injecting via said injection well a drive agent to displacesaid altered fluid and second slug and reservoir oil through saidreservoir toward said production well, (d) producing said reservoir oilvia said production well.
 2. The method of claim 1 wherein said lighthydrocarbon is selected from the group of light hydrocarbons having from2 to 6 carbon atoms per molecule, and mixtures thereof.
 3. The method ofclaim 2 wherein said light hydrocarbon is ethane, propane, LPG, butane,pentane, hexane and mixtures thereof.
 4. The method of claim 1 whereinsaid light hydrocarbon is injected at a rate in the range of about 0.5ft/day to about 15 ft/day.
 5. The method of claim 1 wherein said firstslug is injected in amounts of from about 0.02% to about 5% pore volume.6. The method of claim 1 wherein said second slug comprises carbondioxide and contains inert gas, wherein the ratio of inert gas to carbondioxide is the ratio at which said slug is conditionally miscible withsaid mixture of altered reservoir fluid.
 7. The method of claim 6wherein said inert gas is selected from the group consisting of methane,natural gas, separator gas, flue gas, air, nitrogen and mixturesthereof.
 8. The method of claim 7 wherein said second slug comprisescarbon dioxide and contains light hydrocarbon, wherein the ratio oflight hydrocarbon to carbon dioxide is the ratio at which said slug isconditionally miscible with said mixture of altered reservoir fluid. 9.The method of claim 8 wherein said light hydrocarbon is selected fromthe group consisting of ethane, propane, LPG, butane and mixturesthereof.
 10. The method of claim 1 wherein said second slug is injectedin amounts of from about 10% to about 30% pore volume.
 11. The method ofclaim 1 wherein said second slug is injected at a rate in the range ofabout 0.03 ft/day to about 10 ft/day.
 12. The method of claim 1 whereinsaid drive agent is selected from the group consisting of air, nitrogen,combustion gas, separator gas, natural gas, methane, and mixturesthereof.
 13. The method of claim 1 wherein said drive agent is water,said water being fresh water, brine, reservoir water, thickened waterand mixtures thereof.
 14. The method of claim 13 wherein said watercontains surfactants, thickeners, and mixtures thereof.
 15. The methodof claim 1 wherein said injection is at the top of said reservoir, saiddisplacement is in a substantially vertical direction downward and saidproduction is from the bottom.